The present invention relates to a method for limiting the coating width and/or preventing damage to roll ends while coating or surface-sizing paper or an equivalent web material, in a coating device in which method a coating agent, such as size, coating paste or equivalent, is introduced into the coating device to an application zone therein defined by a coating member, a moving base, lateral seals, and a front wall. The coating agent is spread onto the moving base, such as a roll face or a paper or board web that is passed over the roll, by means of the coating member which is loaded against the moving base.
The present invention also relates to an apparatus for limiting the coating width and/or preventing damage to the roll ends while coating or surface-sizing paper or an equivalent web material in a coating device which comprises an application zone defined by a coating member, a moving base, such as a roll face or a paper or board web that is passed over the roll, lateral seals, and a front wall. A coating agent, such as size, coating paste or equivalent, is introduced into the application zone. The coating member is loaded against the moving base and is arranged to spread and smooth the coating agent onto the moving base.
While coating and surface-sizing paper or an equivalent web material, two alternative methods and devices are commonly used, i.e., a blade coater or a bar coater. In particular in the surface-sizing technique, bar coaters have been used increasingly in recent years, because their usability has proved very advantageous. In coating and surface-sizing, a bar coater is used for spreading and smoothing the coating agent onto the face of the paper or of an equivalent web material, the coating agent being usually applied to the web face by means of an applicator roll. In film size presses, the coating agent may also be applied directly onto the faces of the size-press rolls, from which rolls it adheres in the roll nip to the web running through the nip. As stated above, either a revolving coating bar or a coating blade can be used as the coating member.
When the coating agent is applied, e.g., by means of a grooved coating bar onto the rolls in a size press, the end areas of the roll (also referred to as roll end areas) placed outside the web must be doctored clean by means of lateral doctors. Otherwise, any coating agent remaining in the end areas of the rolls would cause considerable splashing in the roll nip. The doctoring of the lateral areas has proved very difficult, and it also abrades the roll face to a considerable extent. With the introduction of pigmenting techniques with a pigment having a high dry solids content, multiple problems have been encountered in the lateral areas on the rolls. The conventional lateral jets intended for surface sizing have usually been placed in a location in which they have been blocked readily, because the coating paste flows and splashes onto them. If the lateral jets are blocked, lubrication of the lateral areas is prevented, as a result of which, the end areas of the rolls are easily damaged.
It is a further, highly significant drawback of the conventional constructions that the lubrication water coming from the lateral jets passes directly into the recirculation system and conduits of the coating agent, where it dilutes the coating paste. In pigmenting with coating pastes of a high dry solids content, the diluting waters must be minimized. Since the quantity of lubrication water must be minimized, the coating width cannot be effectively controlled and limited sufficiently well. As a result, it has been necessary to employ lateral doctors which abrade the roll ends to a considerable extent, as was stated above. Another disadvantage is that the lateral doctors do not operate reliably with coating pastes or agents having high dry solids contents. Also, small quantities of water produce drying of the paste on the roll face, which causes heating of the coating bar in bar coaters and heating of the coating blade in blade coaters. The heating of the coating member, e.g., coating bar or coating blade, may result in damage to the bar cradle and/or to the roll coating in the lateral areas.